1. Field of the Invention
The invention relates to programmable integrated circuits, and in particular, to an electrically programmable fuse.
2. Related Art
It is often beneficial for an integrated circuit (IC) to include one or more one-time programmable elements (fuses). For example, static random access memories (SRAMs) often include metal fuses that can be blown open with a laser to activate redundancy circuitry, thereby improving production yields. As another example, programmable logic devices (PLDs), such as a field programmable gate array (FPGA), may also include fuses for activating redundant resources, storing encryption or security keys, or other purposes. A PLD may include configurable resources, such as configurable logic, a configurable interconnect structure, programmable input/output blocks, memories, transceivers, and processors. Fuses in a PLD may be used to control some or all of the configurable resources.
However, since an external programming tool (e.g., a laser) can sometimes be cumbersome, it is often desirable for fuses to be electrically programmable. An electrically programmable fuse is typically a device that undergoes a permanent change in electrical characteristics in response to a high voltage or current.
For example, commonly owned U.S. Pat. No. 6,496,416, issued Dec. 17, 2002 to Kevin T. Look, describes a metal-oxide-semiconductor (MOS) device that includes a gate heating element. When a programming voltage is applied across the gate heating element, the gate heating element raises the temperature of the device in the channel region, which results in dopant migration. This dopant migration changes the threshold voltage of the device, thereby allowing programmed and unprogrammed devices to be differentiated.
Because this type of electrically programmable fuse can be formed using standard CMOS process steps, it can be readily integrated into an IC. However, the high temperature provided by the gate heating element can sometimes cause a break in the gate heating element prior to sufficient dopant migration to program the fuse. Reducing the temperature of the gate heating element to prevent this failure can slow dopant migration significantly, which in turn increases both the time required for programming the fuse and the testing costs for the IC that includes the fuse.
Accordingly, it is desirable to provide a reliable fuse that can be efficiently programmed, and that can be manufactured using standard metal-oxide-semiconductor (MOS) transistor structures and semiconductor processes.